The control of the filling degree of a drum debarker has traditionally for decades been based on visual monitoring through the outlet of the drum. At late 1980""s, attempts were made to utilize the log weighing information in controlling the filling degree of the debarking drum. However, it has been difficult to implement a reliable weighing system, since among other complications, the weight of log material in the drum does not necessarily always correlate with the percentage of the drum filling degree. This is because the greenness degree of wood and variations in the moisture content thereof affect the weighing result. Furthermore, variations in the log length and diameter cause changes in the average density of the log bunch in the drum and, hence, affect the weighing result.
An important factor in the function of a drum debarker is the debarking retention time. A longer retention is needed when logs of inferior debarking qualities must be handled. These kinds of factors deteriorating the debarking efficiency are, e.g., drying or freezing of logs. The retention time varies according to the actual wood content of the drum, i.e. the average density of the log bunch, and the drum filling degree. Herein, the term drum filling degree is defined as the ratio of the cross-sectional segment occupied by the log bunch in the overall cross-sectional area of the drum periphery to half the cross-sectional area of the drum periphery, whereby the ratio is conventionally expressed as a percentage. The required retention time is shorter for a high degree of greenness and moisture content of the logs, and frequently for massive logs, too, inasmuch the latter easily augments the contact pressure imposed on the log surfaces to exceed the limit pressure required for dislodging the bark.
It is generally known that short, massive logs give a high average log bunch density even in a tumbling bunch of logs. In contrast, long and slender logs may give a bunch density reaching only 20% of the values achievable by the first kind of log. Resultingly, it is obvious that the debarking process cannot be controlled based on the top level of the log bunch or the weight of logs in the debarking drum. Superficial field experience of these facts has generally forced to adapt a so-called xe2x80x9crunning by a formulaxe2x80x9d convention, whereby the operator of the debarking process has given set values for the drum operation according to the assessed running situation. This approach is exemplified in, e.g., patent publication WO 9117030 according to which the operator enters log parameter information to a drum debarking process control based on weighing data. Herein, a problem arises therefrom that the operator does not have exact information on the current wood species and log type, both parameters typically tending to change very rapidly.
Logs to be debarked have the following parameters that can substantially modify their debarking behavior:
a) Length of log to be debarked, whereby this parameter affects the density of the log bunch tumbling in the drum according to formula (L/l)(xc2xd) so that-short logs (l) give a higher average density of log bunch.
b) The diameter of the log being debarked, whereby the average density of the log bunch tumbling in the drum is affected according to formula (D/d)(⅓) so that massive logs (D) give a higher average density of the log bunch.
c) The greenness or solids content of logs that may affect the specific density of the logs by xc2x120%.
In consideration to the above variations in the average density of the log bunch within the limits of conventional log material to be debarked, it is easy to understand that the drum filling degree can vary widely even when the overall weight of the drum contents is kept constant. It is obvious from the foregoing that a proper retention time based drum control is not achievable solely on the information of the height or weight of the log bunch in the drum. Measurement techniques based only on the height or weight of the log bunch in the drum are disclosed in Finnish patent applications 991530 and 923944.
The method according to the invention now offers a novel technique for controlling the debarking process discussed above in an improved manner utilizing information obtained from drum weighing. The characterizing features of the method are disclosed in appended claim 1.
By virtue of controlling the debarking process using the drum rotating torque information delivered by the drum operating system in combination with the drum weighing information, the control system can compute information on the average density and top level of the log bunch tumbling in the drum. Then, based on this information, it is possible to control the actual mass of logs in the drum. The moisture content, as well as the length and diameter variations of logs can be assessed on the basis of the specific weight of the wood material. This makes it possible to identify required changes in the retention time and perform suitable compensating actions with the help of, e.g., a variable drum rotation speed control that has recently been widely adopted in the industry. Then, it is appropriate to replace the concept of debarking retention time with the concept of debarking work which is proportional to the number of tumbling turns of the log bunch in the drum.